It is usually desirable to include in medium voltage electrical networks devices for detecting the magnitude or presence of voltages on the network, lines or cables. Such a device could either simply indicate the presence of a voltage or it could provide a signal to a remote indication, control or protection device or both. One such type device used to accomplish these functions is a medium voltage capacitive voltage divider which is electrically connected in series with a low voltage impedance combining, for example, a resistor, capacitor, light bulb, relay or transformer windings, or any combination thereof; and which series combination is electrically connected between one of the medium voltage network phases and ground.
Medium voltage capacitors commonly include a ceramic dielectric member having a high specific inducting power, which is encapsulated within a mass of insulating, thermal hardenable resin which in turn forms another element of a medium voltage system such as, for example, a supporting insulator, a current transformer, a passage insulator, etc. A known technique is schematically shown in FIG. 1, where a bus 1 for distributing electrical power at medium voltages, for example 20 KV, is spaced from ground 2 by a supporting insulator which includes a capacitor 3 having a very high capacity which is able to withstand the voltage applied to the bus 1. A resistance 4 is electrically connected in parallel with a signaling indicator 5, this parallel combination being electrically connected in series with the capacitor 3. Similarly, another example of a supporting insulator includes a capacitor 6, placed in series with a winding 7 of a relay or a transformer.
FIG. 2 depicts one type of a medium voltage capacitor, usable as capacitor 3 or 6 in the circuit shown in FIG. 1, which comprises a ceramic dielectric member 8 which is characterized by a very high specific inducting power. The ceramic member 8 is cast within a mass of insulating, thermal hardenable resin 9, which forms the mechanical element for the particular electrical device contemplated, namely a support, current transformer, passage insulator, etc. The member 8 is located between two inserted elements 10 and 11 which are cast in the resin 9. The inserted elements 10 and 11 are electrically conductive, preferably made of metal, and include threads 12 and 13. These inserted elements serve not only as supports but also as terminals and as plates of the capacitor.
In FIG. 3 there is shown another embodiment of a medium voltage capacitor wherein the ceramic element 8 is replaced by two cylindrical electrodes 14 and 15 which are formed from metal sheets or screens and which are maintained at a predetermined distance from each other during the casting of the resin mass 16. The electrode 14 is connected to an inserted element 10 having internal threads 12 while the electrode 15 is connected to an inserted element 11 having internal threads 13.
Both of the prior art embodiments shown in FIGS. 2 and 3 have the disadvantage of developing very high electrical stresses in certain regions of the cast mass as well as very high mechanical stresses due to the differing coefficients of expansion of the elements forming the device. In the case of the embodiment set forth in FIG. 3, the high electric stresses are reduced by means of rings 17 and 18 whose curvatures are calculated in a well known manner in order to achieve acceptable stresses in zones exposed to peak effects. Nevertheless, the problem exists of maintaining the desired relative positions of the two electrodes 14 and 15 in order to insure that they are perfectly concentric. Since this effect is rather difficult to achieve, the known capacitor shown in FIG. 3 has relatively poor reliability.
The object of the present invention is to remedy the foregoing difficulties and provide new and improved capacitive voltage dividers which are more dependable than heretofore, less susceptible to high voltage breakdown due to arc-over and less cumbersome, and which are adapted for manufacture economically in large volume.